De Synodis
57 Though we condemn a plurality of gods and declare that God is only one, we cannot deny that the Son of God is God. Nay, the true character of His nature causes the name that is denied to a plurality to be the privilege of His essence. The words, Beside Me there is no God, cannot rob the Son of His divinity: because beside Him who is of God there is no other God. And these words of God the Father cannot annul the divinity of Him who was born of Himself with an essence in no way different from His own nature. The Jews interpret this passage as proving the bare unity of God, because they are ignorant of the Only-begotten God. But we, while we deny that there are two Gods, abhor the idea of a diversity of natural essence in the Father and the Son. The words, Beside Me there is no God, take away an impious belief in false gods. In confessing that God is One, and also saying that the Son is God, our use of the same name affirms that there is no difference of substance between the two Persons.
XXIV. “If any man says that the Son was made by the will of God, like any object in creation: let him be anathema.”
58. To all creatures the will of God has given substance: but a perfect birth gave to the Son a nature from a substance that is impossible and itself unborn. All created things are such as God willed them to be: but the Son who is born of God has such a personality as God has. God's nature did not produce a nature unlike itself: but the Son begotten of God's substance has derived the essence of His nature by virtue of His origin, not from an act of will after the manner of creatures.
XXV. “If any man says that the Son was born against the will of the Father: let him be anathema. For the Father was not forced against His own will, or induced against His will by any necessity of nature, to beget His Son; but as soon as He willed, before time and without passion He begot Him of Himself and showed Him forth.”
59. Since it was taught that the Son did not, like all other things, owe His existence to God's will, lest He should be thought to derive His essence only at His Father's will and not in virtue of His own nature, an opportunity seemed thereby to be given to heretics to attribute to God the Father a necessity of begetting the Son from Himself, as though He had brought forth the Son by a law of nature in spite of Himself. But such liability to be acted upon does not exist in God the Father: in the ineffable and perfect birth of the Son it was neither mere will that begot Him nor was the Father's essence changed or forced at the bidding of a natural law. Nor was any substance sought for to beget Him, nor is the nature of the Begetter changed in the Begotten, nor is the Father's unique name affected by time. Before all time the Father, out of the essence of His nature, with a desire that was subject to no passion, gave to the Son a birth that conveyed the essence of His nature.
XXVI. “If any man says that the Son is incapable of birth and without beginning, speaking as though there were two incapable of birth and unborn and without beginning, and makes two Gods: let him be anathema. For the Head, which is the beginning of all things, is the Son; but the Head or beginning of Christ is God: for so to One who is without beginning and is the beginning of all things, we refer the whole world through Christ.”
60. To declare the Son to be incapable of birth is the height of impiety. God would no longer be One: for the nature of the one Unborn God demands that we should confess that God is one. Since therefore God is one, there cannot be two incapable of birth: because God is one (although both the Father is God and the Son of God is God) for the very reason that incapability of birth is the only quality that can belong to one Person only. The Son is God for the very reason that He derives His birth from that essence which cannot be born. Therefore our holy faith rejects the idea that the Son is incapable of birth in order to predicate one God incapable of birth and consequently one God, and in order to embrace the Only-begotten nature, begotten from the unborn essence, in the one name of the Unborn God. For the Head of all things is the Son: but the Head of the Son is God. And to one God through this stepping-stone and by this confession all things are referred, since the whole world takes its beginning from Him to whom God Himself is the beginning.
XXVII. “Once more we strengthen the understanding of Christianity by saying, If any man denies that Christ, who is God and the Son of God, existed before time began and aided the Father in the perfecting of all things; but says that only from the time that He was born of Mary did He gain the name of Christ and Son and a beginning of His deity: let him be anathema.”
61. A condemnation of that heresy on account of which the Synod was held necessarily concluded with an explanation of the whole faith that was being opposed. This heresy falsely stated that the beginning of the Son of God dated from His birth of Mary. According to evangelical and apostolic doctrine the corner-stone of our faith is that our Lord Jesus Christ, who is God and Son of God, cannot be separated from the Father in title or power or difference of substance or interval of time.
62. You perceive that the truth has been sought by many paths through the advice and opinions of different bishops, and the ground of their views has been set forth by the separate declarations inscribed in this creed. Every separate point of heretical assertion has been successfully refuted. The infinite and boundless God cannot be made comprehensible by a few words of human speech. Brevity often misleads both learner and teacher, and a concentrated discourse either causes a subject not to be understood, or spoils the meaning of an argument where a thing is hinted at, and is not proved by full demonstration. The bishops fully understood this, and therefore have used for the purpose of teaching many definitions and a profusion of words that the ordinary understanding might find no difficulty, but that their hearers might be saturated with the truth thus differently expressed, and that in treating of divine things these adequate and manifold definitions might leave no room for danger or obscurity.
Source: On the Councils, or the Faith of the Easterns (New Advent)